Weblate

Weblate provides a Libre software project license, and now supports Fluent.

I've set up a Weblate project for portmod: https://hosted.weblate.org/projects/portmod/

I think we may as well deprecate the use of pontoon. While pontoon better integrates editing fluent messages into its UI, I've only just noticed that we don't get any sort of notifications from our pontoon instance since email was never set up for it (and there were a dozen pending translations). While I could set that up, I feel like hosted.weblate.org is probably better in the long run since it provides a larger community of translators, and requires less maintenance (the Pontoon instance was also broken and I just spent a while figuring out how to get it working so that the pending translations could be synchronized).

I've also restructured the "Development Guide" in the docs to make Localisation a little easier to find. "Portmod Development Guide" has been changed to "Contributing to Portmod", and the subcategories have been flattened out so that "Package Development", "Localisation" and "Developer Setup" are clearly displayed right under the Contributing to Portmod category.